In wiring integrated circuits, selected metal deposits are placed on substrates containing devices. These metal deposits serve to electrically connect discrete devices formed in the substrate and may be created, for example, by chemical vapor deposition processes or other such techniques that are well known to the semiconductor art. These deposits may also be in the form of lines deposited over and through insulating layers and have other insulating layers formed over them. In most cases, these result in an uneven surface which must then be planarized by removal of some of the over lying materials. In other cases the deposits must be configured by a partial removal of the deposited material. Such removal is usually done by the use of chemical/mechanical processes using lapping or polishing machines. In these machines, selected amounts of material must be removed to provide a desired surface without removing excess amounts of desired underlying materials, thus precise endpoint detection techniques are required.
Presently, there are various types of machines for polishing, removing or reducing layers on the surface of semiconductor wafers. In general such machines include top and bottom plates between which the wafers to be treated are positioned. The two plates are moved relative to each other and a slurry consisting of an abrasive solution, with or without an etching reagent, is fed between the plates to grind and flush away the wafer particles. Such machines are well known to the art and are commercially available.
In such machines, various devices and methods have been used for determining endpoints in the processing of such semiconductor wafers. U.S. Pat. No. 3,874,959, discloses an apparatus for detecting the etch endpoint of an oxide coated semiconductor substrate using electrodes coupled to the semiconductor wafer and to the solution used to etch the oxide so that as the oxide is etched away a signal is detected by virtue of the etching solution making electrical contact with the underlying substrate material.
U.S. Pat. No. 4,207,137 discloses a plasma endpoint etching process wherein the impedance of the plasma varies during the etching process which variation can be measured and the endpoint determined thereby.
U.S. Pat. No. 4,602,981, also determines endpoint in a plasma etching process by measuring the RF voltage of an electrode in a plasma etching process.
IBM Technical Disclosure Bulletin, Vol. 31, No. 4, September 1988, on pages 325 and 326, describes an endpoint detector for a chemical/mechanical polisher wherein the current to the motor driving the table is monitored to determine when a particular layer has been removed and the material underneath it is being polished.
Another method to determine endpoint in polishing machines is described in U.S. Pat. No. 4,910,155 wherein a monitor wafer is coated with the desired oxide thickness and the current of the motor driving the wafers is continuously measured until the oxide on the dummy wafer is totally removed causing a current spike in the motor driving the dummy wafer. The detected current spike is an active indication as to when the oxide material has been removed from the monitor wafer and the process may be then terminated. Other wafers being simultaneously lapped and having an initial oxide thickness greater than the oxide thickness of the dummy wafer will be left with an oxide thickness which is the difference between their original thickness and the thickness removed from the monitor wafer.
This method relies upon a massive frictional drag being realized, by an abrupt change from a soft, easily polished material, such as, oxide to a hard, difficult to polish material, such as silicon.
There remains a continuing need in the semiconductor fabrication art for improved apparatus and methods for accurately and efficiently detecting the endpoint of a lapping, planarization process wherein there is an abrupt change from a difficult to polish material to a more easily polished material.